The present invention relates to a vertical external cavity surface emitting lasers (xe2x80x9cVECSELsxe2x80x9d) excited by optical or electrical pumping. More particularly, the present invention relates to VECSELs which are tunable, which may be made stable at one of a selectable plurality of modes, and which are more readily fabricated and with less complexity than prior approaches.
Practical semiconductors lasers generally follow two basic architectures. The first laser type has an in-plane cavity, and the second laser type has a vertical cavity, a so-called vertical-cavity surface-emitting laser or xe2x80x9cVCSELxe2x80x9d. If the optical resonance cavity is formed externally of the semiconductor structure, the laser is known as a vertical external cavity surface-emitting laser or xe2x80x9cVESCELxe2x80x9d.
One known drawback of in-plane diode lasers, and most particularly the Fabry-Perot type, is that it has a tendency to mode-hop, i.e. to hop unpredictably to a different mode (wavelength) in spite of a constant pumping current. As the current is increased, there are wavelengths at which the mode hopping becomes uncontrollable. Moreover, diode lasers may show hysteresis, in that mode hopping may occur at different wavelengths during control current increases than at control current decreases. Another issue with in-plane diode lasers is the transverse optical beam profile is typically elliptical rather than circular, and has high divergence, increasing the complexity of coupling the laser energy into an optical fiber. Precision gluing of tiny aspheric lenses at the laser-fiber interface is often required.
Furthermore, such lasers only have about 30 to 35 dB of side mode suppression. If the side modes are not well enough controlled, the laser may excite two or three adjacent communications channels, resulting in unwanted interference.
VCSELs include semiconductor structures which have multiple layers epitaxially grown upon a semiconductor wafer or substrate, typically gallium arsenide. The layers comprise Bragg or dielectric-layer mirrors which sandwich layers comprising quantum well active regions. Within the VCSEL, photons emitted by the quantum wells are reflected between the mirrors and are then emitted vertically from the wafer surface. VCSEL lasers typically have a circular dot geometry with lateral dimensions of a few microns. The emitting aperture of a few microns facilitates direct-coupling to optical fibers or other simple optics, since a narrow aperture typically supports only a single lateral mode (TEM00) of the resulting optical waveguide, but is sufficiently wide to provide an emerging optical beam with a relatively small diffraction angle. The typical power does not exceed 3 mW in TEM00. Recently, a 1.3 micron VCSEL was said to be developed by Sandia National Laboratories in conjunction with Cielo Communications, Inc. According to a news report, xe2x80x9cThis new VCSEL is made mostly from stacks of layers of semiconductor materials common in shorter wavelength lasers . . . aluminium gallium arsenide and gallium arsenide. The Sandia team added to this structure a small amount of a new material, indium gallium arsenide nitride (InGaAsN), which was initially developed by Hitachi of Japan in the mid 1990s. The InGaAsN causes the VCSEL""s operating wavelength to fall into a range that makes it useable in high-speed Internet connections.xe2x80x9d (xe2x80x9cFirst ever 1.3 micron VCSEL on GaAsxe2x80x9d, Optics.Org Industry News, posted Jun. 16, 2000.) One of the characteristics of VCSELs is that the laser cavity is formed entirely within the semiconductor structure.
As mentioned above, if a cavity is formed which is external to the semiconductor structure having the quantum well active region, the laser is known as a VECSEL. One example of an optically-pumped VECSEL is described in IEEE Photonics Technology Letters Vol. 9, No. 8 pp 1063-1066 and in WO 00/10234, the disclosures thereof being incorporated herein by reference. The disclosed VECSEL includes an epitaxially-grown semiconductor structure or chip having a multiple-layer mirror structure integrated with a multiple-layer quantum-well structure which provides a gain medium, and an external mirror forming a resonant cavity with the integrated semiconductor multilayer mirror. Optical pumping radiation is directed at the quantum-well and pump-absorbing layers. The quantum-well layers release photons in response to the pumping energy, and the external cavity is dimensioned to result in laser energy output at approximately 976 nm in response to pumping energy at approximately 808 nm. Because this VECSEL operates in the visible light spectrum, the active gain medium is made to be aluminum-free, since aluminum ions tend to diffuse in visible light lasers. Accordingly, the quantum-well and pump-radiation absorbing layers are aluminum-free layers of alloys of gallium arsenide and indium gallium arsenide phosphide (GaAs/InGaAsP).
One approach for tuning a VECSEL is described in a paper by D. Vakhshoori, P. Tayebati, Chih-Cheng Lu, M. Azimi, P. Wang, Jiang-Huai Zhou and E. Canoglu entitled, xe2x80x9c2 mW CW single mode operation of a tunable 1550 nm vertical cavity surface emitting laser with 50 nm tuning rangexe2x80x9d, published in Electronics Letters, Vol. 35, No. 11, May 27, 1999, pp. 1-2, the disclosure thereof being incorporated herein by reference. The VECSEL structure described in this note comprises an indium phosphide substrate carrying an epitaxially-grown 1.55_m multiple quantum well system. A via is formed through the bottom of the substrate and a thermally conductive multilayer mirror is deposited into the via to form the bottom mirror of the cavity. A support post structure and a top membrane having a multilayer top mirror structure is formed on top of the active region. The radius of curvature of approximately 300 xcexcm of the top mirror results in a stable optical resonator cavity as well as a pumping-exit window. To achieve tuning, a voltage is applied between the top membrane and the bottom mirror. The electrostatic force generated will pull the top mirror toward the bottom mirror, reducing the cavity length and also reducing the laser wavelength. With a 980 nm laser pump at 40 mW, a TEM00 single mode output at approximately 2 mW is achieved by the VECSEL. A tuning voltage from 0 volts to 40 volts changes the VECSEL""s output wavelength from 1564 to 1514 nm. One drawback of the VECSEL described in this note is the fabrication complexity. Another drawback is that the VECSEL must be continuously regulated by a voltage control loop in order to maintain the VECSEL at the desired wavelength.
VECSELs may have as a gain structure a few microns thick multiple quantum well active region sandwiched between a bottom Bragg minor grown on a semiconductor substrate, and an epitaxially grown antireflection coating or dielectric coating. An external high reflectivity dielectric concave minor is then added to form an external optical cavity. Co-inventors Garnache, Kachanov and Stockel of the present invention have previously reported in a paper entitled xe2x80x9cHigh-sensitivity intracavity laser absorption spectroscopy with vertical-external-cavity surface-emitting semiconductor lasersxe2x80x9d, Optics Letters, Vol. 24, No. 12, Jun. 15, 1999, pp. 826-828 (the disclosure of which is incorporated herein by reference),that an optically pumped multiple-quantum-well (xe2x80x9cMQWxe2x80x9d) VECSEL is an excellent candidate for use in high sensitivity intracavity laser absorption spectroscopy (xe2x80x9cICLASxe2x80x9d). In the ICLAS method an absorbent analyte is placed inside an external cavity of a broadband laser with homogeneously broadened gain. In the setup reported in this paper, a VECSEL was grown by molecular beam epitaxy on a 0.5 mm gallium arsenide substrate. The bottom stack was a standard Bragg mirror comprising 30.5 pairs of aluminum arsenide/aluminum gallium arsenide quarter-wave layers having a measured reflectance of 99.96 percent at a design wavelength of 1030 nm. The MQW active region comprised two sets of three (2xc3x973) strained 8 nm indium gallium arsenide quantum wells separated by 10 nm gallium arsenide baffler layers. Each set was placed at the maximum of the intra-cavity standing wave. A quarter-wave layer of aluminium arsenide was grown on top of the active region followed by an aluminium gallium arsenide (Al007Ga093As) half-wave layer. This layer, which has a higher bandgap energy than the active zone material GaAs is needed to prevent carriers from diffusing to the semiconductor surface and to have an aluminum-poor surface to avoid surface deterioration. In this arrangement the air interface/Bragg minor sub-cavity formed by the semiconductor structure operates at anti-resonance. Reference may also be made to IEE Photonics Technology Letters Vol. 8 No. 3 March 1996 pp313-315, Sandusky and Brueck (the disclosure of which is incorporated herein by reference). That paper discloses a VECSEL having an active region between a standard high reflector and an anti-reflection (AR) layer. In spite of the AR layer, and because the active region is of great length (it includes thirty quantum wells), the subcavity operates at resonance and the laser therefore suffers from problems similar to those referred to above.
In one aspect of the present invention, unsolved problems can be overcome by providing a semiconductor structure forming a sub-cavity operating at anti-resonance to achieve a broadband gain medium and/or a homogeneously broadened active gain medium. We have achieved this desired antiresonance by matching the behaviour of an antireflective layer with the behaviour of the whole of the sub-cavity, in particular by correlating the bandwidth of the antireflective coating with the free-spectral range of the sub-cavity. As a result of this correlation, the antireflectant layer may be regarded as an anti-resonance layer.
We have now designed a new combination of high reflectance mirror (such as a Bragg mirror), active region (which may contain one or more quantum wells) and an anti-reflection component (AR) (generally a coating, and may have the form of a Bragg stack). In general, this combination may be regarded as a gain mirror. This new design may increase the spectral bandwidth of the coupling between the active region and the external laser cavity by using an AR coating that is narrow-band and by designing the sub-cavity of the gain mirror so that it operates at anti-resonance. It preferably uses an active region that is thinner than relevant prior-art designs.
The narrow-band AR coating is preferably a Bragg stack comprising a multiplicity of dielectric layers. The reflection at the gain mirror interface with the air (i.e. at the interface of AR coating with the air in the external cavity) is significantly larger than the reflection at interfaces between the layers in the active region so the space between the surface of the AR coating at the air interface and the highly reflective Bragg stack at the opposite end of the gain mirror acts as a sub-cavity. The sub-cavity works at anti-resonance at the design wavelength xcex (see below), and hence the sub-cavity length should be an odd multiple of xcex/4. However, the narrow-band response of the AR stack results in a resonance condition being set up at certain wavelengths either side of xcex, namely where the sub-cavity length is an even multiple of that other wavelength divided by four. Since the effective gain is proportional to the product of the material gain produced by the quantum wells in the active region and the modulus squared of the electric field [E]2, the net effect of the AR stack is to increase the effective gain bandwidth of the gain mirror. The AR stack bandwidth is designed to match approximately the free spectral range (FSR) (see below) between the two sub-cavity modes i.e. the mode associated with anti-resonance and the mode associated with resonance. As a result the gain mirror has a filter profile with a shape like a top-hat and it is considerably broader than the single-peak curve characteristic of the intrinsic gain bandwidth.
This new design for the gain mirror has the advantage that it increases the gain bandwidth and hence improves coupling between the sub-cavity and the external cavity in the region of the spectrum close to the design wavelength. The multilayer structure of the AR coating is specially designed to provide such coupling between the sub-cavity and the external cavity at wavelengths close to the design wavelength.
One object of the present invention is to realise a sub-cavity operating an anti-resonance and an antireflection coating.
A further object of the present invention is to realise an optically or electrically pumped semiconductor laser having a broadband gain medium and/or a homogeneously broadened gain medium with spectral narrowing as a function of operating time.
Another object of the present invention is to provide a VECSEL laser having a sub-cavity operating at anti-resonance which may readily be adapted and used in diverse applications including ICLAS, cavity ring down spectroscopy (xe2x80x9cCRDSxe2x80x9d) and optical fiber-based telecommunications systems and ultra-short pulse operation by modelocking.
Another object of the present invention is to realise a VECSEL having a semiconductor structure formed by molecular beam epitaxy in a manner obviating the need for a semiconductor substrate, thereby overcoming limitations and drawbacks of prior approaches in which a substrate contributed to the presence of a Fabry-Perot etalon or other unwanted optical element.
Another object of the present invention is to realise a laser operating reliably and solely at wavelengths employed in standardized wavelength division multiplexing (WDM) used in optical fiber telecommunications networks. Thus, the laser is able to mode-hop precisely from channel to channel with an accuracy better than ten percent of the channel spacing.
One more object of the present invention is to realise an optically pumped or electrically pumped MQW VECSEL having sidemode suppression well in excess of 50 dB.
Thus, the invention provides a laser comprising;
(1) the following layers in the following order
(a) a mirror such as a Bragg mirror in particular a hybrid metal/Bragg mirror;
(b) an active region providing optical gain; and
(c) an anti-reflection coating which together with the active region defines a sub-cavity; and
(2) a second mirror spaced from coating (c) to define an external cavity;
the layers (1) being such that the free spectral range of the sub-cavity is less than 2 times particularly less than 1.5 times especially less than one times the bandwidth of the coating (c);
and the bandwidth of the mirror (a) is at least at great, preferably at least 1.5 time as great, as the free spectral range of the sub-cavity.
The active region (b) preferably has at least one, more preferably from one to twenty quantum wells. For many purposes, the preferred number will be from two to ten, especially from four to ten quantum wells. The quantum wells are desirably located at peaks in an e-field distribution at a design wavelength of the laser. The term xe2x80x9cdesign wavelengthxe2x80x9d will be well-understood by the reader of this specification, and it is in any case defined below.
The mirror (a) and the active region (b) preferably together form a gain mirror. The gain will usually be positive, but may be negative and in that case the various components that form part of the laser defined herein may be used as a filter. The gain mirror and the external cavity preferably have a filter function such that when the laser is tuned over 1% (preferably over 5% especially over 10%) of its center wavelength of the filter function, then a threshold value of the laser remains below 1000% of its minimum value, preferably below 500%, especially below 200%.
Alternatively or additionally, when the laser is tuned over 1% (preferably over 5% especially over 10%) of its center wavelength of the filter function then a conversion efficiency of the laser remains above 10% of its maximum value, preferably above 25%, especially above 50%.
Region (b) will in general be capable of optical gain at some wavelength xcex and we prefer that region (b) has an optical path length of 20 xcex/2 or less, more preferably 10 xcex/2 or less.
We also prefer that the anti-reflection coating has an optical pathlength of 25 xcex/2 or less, and preferably that it comprises the following layers in the following order in a direction away from active region (b):
(i) a layer having an optical pathlength of xcex/4 and having a lower refractive index;
(ii) at least one pair (say from one to twenty especially one to ten usually one to five pairs) of layers each layer being of optical pathlength xcex/4, the layers being of alternating higher and lower refractive indices, the layer closest to layer (i) being of higher refractive index; and optionally
(iii) a layer of optical pathlength xcex/2 having a higher refractive index.
When layer (iii) is present we prefer that the active region (b) comprises gallium arsenide; layer (i) comprises aluminum arsenide; the or each layer of higher refractive index of pair or pairs (ii) comprises aluminum gallium arsenide and the layer or layers of lower refractive index of said pair or pairs comprises aluminum arsenide; and layer (iii) comprises aluminum gallium arsenide.
When the layer (iii) is absent we prefer that the active region (b) is selected from the group consisting of indium gallium arsenide phosphide and aluminum gallium arsenide; layer (i) comprises indium phosphide; and the or each layer of higher refractive index of the pair or pairs (ii) comprises aluminum gallium indium arsenide and the or each layer of lower refractive of said pair or pairs comprises indium phosphide.
It should be noted that all layers of higher refractive index need not have the same refractive index, and all layers of lower refractive index need not have the same refractive index. In referring to higher and lower refractive indices, we are merely making comparisons between mutually adjacent layers. Nonetheless, it is preferred that all layers of higher refractive index have refractive indices higher than the refractive indices of all layers of lower refractive indices.
The invention also provides an external cavity laser, comprising:
(a) a first mirror and
(b) a second mirror which together with the first mirror defines the external cavity;
the first mirror and the external cavity having a filter function such that when the laser is tuned over 1% (preferably over 5% especially over 10%) of a center wavelength of the filter function, a threshold value of the laser remains below 1000% of its minimum value, preferably below 500%, especially below 200%.
Alternatively, or additionally, when the laser is tuned over 1% (preferably over 5% especially over 10%) of a center wavelength of the filter function, a conversion efficiency of the laser remains above 10% of its maximum value, preferably above 25%, especially above 50%.
The invention further provides a laser sub-cavity comprising:
(a) a first region comprising a layer capable of optical gain at a wavelength xcex and having an optical pathlength of 20 xcex/2 or less;
(b) an anti-reflection coating on the region (a), having an optical pathlength of 25 xcex/2 or less and which comprises the following layers in the following order in a direction away from region (a):
(i) a layer having an optical pathlength of xcex/4 and having a lower refrective index;
(ii) at least one pair (say from one to twenty especially one to ten usually one to five) of layers each layer being of optical pathlength xcex/4, the layers being of alternating higher and lower refractive indices, the layer closest to layer (i) being of higher refractive index; and optionally
(iii) a layer of optical pathlength xcex/2 having a higher refractive index.
The materials referred to above in connection with the laser of the invention may be employed in the sub-cavity of the invention.
The laser of the invention may have a laser diode pump or other light source or an electrical pump for activating the active region (b).
The laser of the invention is of particular benefit for use in intra cavity laser absorption spectroscopy (ICLAS) and may therefore have within the external cavity an analyte cell.
xe2x80x9cBandwidthxe2x80x9d as used herein means the width in Hz at half of the peak amplitude of the relevant characteristic.
The free spectral range (FSR) of the sub-cavity, which is the separation between resonances, is given by the expression
FSR(Hz)=C/(2xc3x97L0) 
where C is the speed of light, and L0 in the optical path length, i.e. the length of the sub-cavity times the average refractive index throughout the sub-cavity. A condition of antiresonance will arise when the optical path length is an odd number of quarter wavelengths at the so-called design wavelength of the laser (in effect the wavelength that is at the center of the filter function). That wavelength is desirably close to (say within 20%, especially within 10%) of the wavelength of maximum gain of the active region, and preferably close to (say within 20%, especially within 10%) of the wavelength of maximum reflectance of the Bragg or other mirror. It is desirable that the bandwidth of the antireflective coating be as broad as possible relative the FSR, preferably that it be as least half as broad, more preferably at least as broad as the FSR. Although an antireflective coating of usefully-broad bandwidth was disclosed in the Sandusky and Brueck paper referred to above, it was used in conjunction with a long sub-cavity, and as a result the benefits of the present invention were not obtained.
In accordance with preferred embodiments of the present invention, an opticallyxe2x80x94or electricallyxe2x80x94pumped vertical external cavity surface emission laser having a design wavelength (as defined above) may include a heat sink structure and a semiconductor structure grown for example by molecular beam epitaxy upon a substrate and attached to the heat sink. As completed the semiconductor structure comprises a multi-layer semiconductor mirror region, such as a Bragg mirror (by which term we include hybrid Bragg mirrors, preferably additionally comprising a metal) generally achieving at least 99 percent reflectance, a single, or more usually a multiple, quantum well active gain region having a length equal to at least one design wavelength and having a plurality of quantum wells, each quantum well being optimally positioned with respect to a standing wave (usually at or close to peaks thereof) in the active gain region at the design wavelength, and an antireflection coating region having a low reflectance at the design wavelength. As completed the sub-cavity of the semiconductor structure has an effective length corresponding to an odd number of quarter design wavelengths so as to operate in anti-resonance at the design wavelength. An external spherical or other suitable lens is positioned separated from the semiconductor structure (for example by means of a spacer structure mounted on the heat sink) by for example a distance not in excess of 10 mm to form a vertical external cavity.
Adaptations and versions of this VECSEL are particularly useful in ICLAS and cavity ring-down spectroscopy (CRDS), and in tunable single frequency lasers and mode-locked lasers, for example those suitable for use in optical telecommunications applications. A preferred fabrication method for making VECSELs in accordance with the present invention is also described.